Aging is a major risk factor for developing amyotrophic lateral sclerosis (ALS) (Niccoli et al. 2017). ALS is a neurodegenerative disease characterized by the loss of motor neurons with an unfavorable outcome (< 5% survival at 5 years after diagnosis). Cellular senescence was first described by Hayflick in the 1960s as a limitation on division of normal cells in vitro (Hayflick & Moorhead 1961). The cellular mechanisms behind this phenomenon were later described, with an important role for cell cycle inhibitors, highlighting p16-INK4a as the major contributor (Serrano et al. 1997). Another hallmark of senescent cells is the increase in β-galactosidase, commonly known as SA-β-gal which is associated with an increase in lysosomal biogenesis (Kurz et al. 2000). Cellular senescence has been described as a barrier against oncogenesis, with a tradeoff where these cells can develop a pro-inflammatory status known as SASP. This process reflects an attempt to induce tissue repair in which senescent cells, usually accumulating DNA damage, can stimulate its clearance by the immune system. Regarding neurodegenerative diseases, several groups have independently demonstrated the presence of senescent glial cells and SASP in the central nervous system (CNS).
Another process related to aging is the change in alternative splicing (AS), a conserved mechanism that increases the complexity of the proteome. TDP-43 regulates a large number of AS events in a complex way (Tollervey et al. 2011). Several evidence support the role of TDP-43 pathology in age-related neurodegenerative processes and physiological aging (McAleese et al. 2017). Most AS events regulated by TDP-43 involve the repression of a set of non-conserved (cryptic) exons which are abnormally incorporated into mRNA in ALS (Ling et al. 2015). In this line, we previously quantified the rate of inclusion of cryptic exons in nervous tissue from ALS donors and cellular models and found a positive correlation with age at death (Torres et al. 2018).
To clarify whether senescence-associated phenomena and TDP-43 dysfunction could be implicated in ALS, we measured the abovementioned variables in the familial ALS transgenic mouse model hSOD1-G93A at different disease stages. The senescence markers p16 and p21, typical biomarkers of senescent cells (Coppé et al. 2010), were analyzed in LSC. Using two different technics (IHC and IF), we characterized the cellular expression pattern of p16. The results show that the expression of p16 mRNA was progressively increased during disease evolution (Fig. 1A), whereas p21 mRNA levels were only higher at the end-stage (Fig. 1B). p16 and p21 exhibited a predominantly cytoplasmic pattern (Fig. 1C), in contrast to recent results from an ALS rat model where it was mainly nuclear (Trias et al. 2019). As shown by IF, p16 positive cells were microglia (Iba1 + cells) (Fig. 2A) and astroglia (GFAP + cells) (Fig. 2B). These results indicate dynamic changes in cellular senescence-associated markers and SASP related to disease evolution. p16 expression is highly expressed before the symptomatology in our transgenic mice, similarly to p16 + microglia in LSC from transgenic rats (Trias et al. 2019). Both facts suggest a role for p16 in disease initiation and progression. Interestingly, senescence-associated cell cycle arrest in an early symptomatic stage (120d) is driven exclusively by p16, whereas p21 only increases later in this model. This may be seen as a result of the late-onset activation of p53 and the DNA damage response pathway, similar to what occurs in the senescence process in microglia (Stojiljkovic et al. 2019). In contrast to p21 (related to reversible cell cycle arrest or quiescence), the senescence process depends heavily on prolonged p16 expression. Strictly speaking, our work and most published articles on ‘senescence’ do not demonstrate an always irreversible cell cycle arrest. There may be divergent processes sharing common biomarkers (Sharpless & Sherr 2015). This is the case with macrophage polarization, in which p16 expression and SA-β-gal activity are physiological, reversible, and not associated with cellular senescence (Hall et al. 2017). In this line, cytoplasmic p16 can regulate cell migration in a manner similar to cyclin D1 (Chen et al. 2013). This evidence reflects a convergent pathway of cell cycle- and senescence-associated proteins regulating cytoskeleton functions. In the case of ALS, cytoskeleton regulators like Rac1 and Cdc42 are implicated in the disease progression and neuroinflammation (D’Ambrosi et al. 2014). Thus, we hypothesized that cytoplasmic p16 could have a similar role in ALS. Like p16 cytoplasmic functions, p21 inhibits the ROCK/LIMK/Cofilin Pathway through MAPK signaling, inducing cytoskeleton remodeling (Tanaka et al. 2002)
We also analyzed another senescence canonical biomarker: SA-β-gal activity. The main cellular populations expressing SA-β-gal in ventral LSC are the motor neuron cells (Nissl + cells in the ventral horn, with a motor-neuron compatible cellular size). Neurons of other LSC locations and the vast majority of Nissl- do not show SA-β-gal activity (Fig. 1D and S1). Interestingly, SA-β-gal activity was reduced during disease progression in motor neurons and in a small fraction of Nissl- cells (compatible with glia). Our findings agree with previously shown data demonstrating that SA-β-gal activity in neurons is not associated with senescence, although it is increased in aging mouse brain (Piechota et al. 2016). Our results suggest that motor neurons contain more lysosomes in cell body than other cells, and that their biogenesis is compromised in this ALS mouse model. In this line, lysosomal mass deficit has already described in this model, highlighting a role of hSOD1 aggregates disturbing lysosomal biogenesis (Xie et al. 2015) and potentially explaining our results from the SA-β-gal activity assay.
Another marker commonly employed in senescence description is the increase in cytokines linked to SASP. In this case, we quantified the expression of typical SASP markers Il1a and Il6. We analyzed as well the expression of Ifna and Ifnb (corresponding to type-I IFN response) as they are postulated as late-senescence markers and could be helpful in determining senescence progression in the LSC of this model. The expression of Ifna was not detected in any of the analyzed samples (data not shown). We observed a different pattern of expression between Il1a (Fig. 3A) and Il6 (Fig. 3B). Il1a is increased in the pre-symptomatic stage and is known to be the upstream regulator of IL-6 in SASP (Orjalo et al. 2009). IL-6 is increased in cerebrospinal fluid in ALS, Alzheimer’s, and Parkinson’s disease (Chen et al. 2018). In contrast, Ifnb expression (Fig. 3C) is not altered, which could indicate that senescence in this model does not evolve a late phase. Overall, this might reflect a complex interaction between senescence, SASP, and changes in reactive glial cells and neurodegeneration.
Regarding TDP-43 splicing function, in mice it controls the inclusion in Adipor2 mRNA (Figure S2). In line with loss of TDP-43 function in this model, cryptic exon inclusion in Adipor2 mRNA was higher in lumbar spinal cord in end-stage mice (Fig. 3D) and positively correlated with p16 expression (Fig. 3E). The present data are the first to show specific alteration regarding splicing function in this ALS model. Notably, this process is associated with an increase in the senescence marker p16, and the two processes are likely to be linked in the same pathway.
We wanted to explore the potential benefits of senolytic treatment due to the higher expression of senescence related genes in this mouse model. We performed Navitoclax treatment following the protocol described for Alzheimer’s disease mouse model (Bussian et al., 2018). The treatment was initiated at 90 days old and finished at end point (Fig. 4A). We estimated the disease progression by weight loss. Navitoclax treatment did not prevent weight loss, neither prolonged survival (Fig. 4B and 4C). Finally, we quantified senescence and SASP genes in lumbar spinal cord. None of the analyzed genes showed statistically significant differences (Fig. 4D). These results suggest differences in molecular effectors between Alzheimer’s and ALS.
Navitoclax is an inhibitor of antiapoptotic protein Bcl2 (Zhu et al., 2016). Senescent cells are highly dependent of different antiapoptotic members. Senolysis is achieved when this antiapoptotic protein is inhibited, promoting cell death (Zhu et al., 2015). Navtioclax treatment is not enough to slow the disease progression and does not extend the survival. In contrast with data in Alzheimer’s and Parkinson’s disease models, this treatment does not prevent the increase of senescence and SASP markers. It suggests that senescence phenotype is not driven by Bcl2 expression of stressed or aged cells in this model (Zhu et al., 2015). Further studies are warranted to determine whether senescence-linked phenomena are mechanistically involved in this fatal disease, clearing the pathway for therapeutic development.
In the case of ALS, we speculate that Navitoclax treatment is not efficient because Bcl2 is not overactivated in our G93A mouse model (Vukosavic, Dubois-Dauphin, Romero, & Przedborski, 1999). However, Bcl-XL, a Bcl-2 family member, is overactive in astrocytes and provides pro-survival input and may mediate the activation of toxic astroglia (Lee, Kannagi, Ferrante, Kowall, & Ryu, 2009). It suggests that an specific inhibition Bcl-XL could have greater effects on disease progression.
In conclusion, the LSC from the hSOD1-G93A mouse, a model of familial ALS, exhibits a non-canonical profile of senescence biomarkers. It is characterized by an early increase in p16 and a late increase in p21, with both displaying a mainly cytoplasmic pattern in glial cells without an increase in SA-β-gal activity. In the case of SASP, it also has a dynamic profile with increasing levels of Il1a from the pre-symptomatic stage onward and an acute peak of expression in end-stage transgenic mice. Regarding AS, this tissue shows a dysfunctional splicing activity of TDP-43 in end-stage ALS mice. This is the first time that senescence markers, SASP, and TDP-43-associated splicing dysfunction have been described in this ALS mouse model.